Liquid crystal display devices are widely used as display devices for various information processing apparatuses such as computers and televisions. In particular, TFT-type liquid crystal display devices (hereinafter, also referred to as “TFT-LCDs”) come into wide use, and thus further development of the market thereof is expected. This leads to a demand for much improved image quality. The following description takes a TFT-LCD as an example, but the present invention is not limited to TFT-LCDs and can be applied to the whole liquid crystal display devices, such as liquid crystal display devices of simple matrix type and those of plasma address type.
The mode most widely used in TFT-LCDs up to now is a mode in which liquid crystal having positive anisotropy of dielectric constant is horizontally aligned between substrates opposed to each other, that is, a twisted nematic (TN) mode. TN-mode liquid crystal display devices are characterized in that the alignment direction of liquid crystal molecules adjacent to one substrate is twisted with an angle of 90° with the alignment direction of liquid crystal molecules adjacent to the other substrate. An inexpensive production technique is established for such TN-mode liquid crystal display devices, and the TN-mode liquid crystal display devices are industrially mature. However, it is difficult to achieve a high contrast ratio on TN-mode liquid crystal display devices.
On the other hand, liquid crystal display devices of another mode are known in which liquid crystal having negative anisotropy of dielectric constant is vertically aligned between substrates opposed to each other, in other words, VA-mode liquid crystal display devices. In VA-mode liquid crystal display devices, liquid crystal molecules are aligned in the direction substantially vertical to the substrate surface when no voltage is applied, and thus the liquid crystal cell hardly shows birefringence and optical rotation, so that light passes through the liquid crystal cell with the polarization state thereof being hardly changed. Thus, the display device including a pair of polarizers (linear polarizer) disposed on the top and the bottom of a liquid crystal cell such that the absorption axes of the polarizers are perpendicular to each other (hereinafter, also referred to as “crossed-Nicols polarizers”) can display a substantially perfectly black screen when no voltage is applied. When a voltage not lower than the threshold voltage is applied (hereinafter, abbreviated as “when a voltage is applied”), the liquid crystal molecules are inclined to be substantially parallel with the substrates, thereby showing a high birefringence and displaying a white screen. Accordingly, such VA-mode liquid crystal display devices can easily achieve a very high contrast ratio.
If the liquid crystal molecules are aligned in one direction when a voltage is applied, such a VA-mode liquid crystal display device suffers asymmetry in viewing angle characteristics. Thus, an alignment-division-type VA mode (what is called a multi-domain VA mode (MVA mode)) is widely used in which the liquid crystal molecules are divided into multiple sections with multiple inclination directions in each pixel by, for example, figuring out a good structure of pixel electrodes or disposing alignment-controlling means such as protrusions in pixels.
In order to maximize the transmittance in the state of displaying a white screen, the MVA-mode liquid crystal display device is designed such that the axis azimuth of a polarizer and the inclination azimuth of liquid crystal molecules when a voltage is applied usually form an angle of 45°. This is because the transmittance in the form of disposing a birefringent medium between crossed-Nicols polarizers is proportional to sin2(2α) where α (unit: rad) represents an angle formed by the axis of a polarizer and the slow axis of the birefringent medium. In typical MVA modes, the liquid crystal molecules may be divided into four domains with inclination azimuths of 45°, 135°, 225°, and 315°. Even in such an MVA mode with four divided domains, Schlieren texture or alignment in unexpected directions are observed in many cases at a border of domains or near alignment-controlling means, causing transmittance loss.
In order to solve such problems, a VA-mode liquid crystal display device using a circularly polarizing plate is proposed (for example, see Patent Literature 1). In such a liquid crystal display device, the transmittance with a birefringent medium being disposed between left- and right-handed circularly polarizing plates perpendicular to each other is independent from the angle formed by the axis of a polarizer and the slow axis of the birefringent medium. Thus, even though the liquid crystal molecules are not inclined in an azimuth of 45°, 135°, 225°, or 315°, a desired transmittance can be secured just by controlling the inclination angle of the liquid crystal molecules. Consequently, for example, a circular protrusion may be formed at the center of a pixel so that the liquid crystal molecules are inclined in all the azimuths, or may be inclined in random azimuths without any control of the inclination azimuths. The VA mode using a circularly polarizing plate herein is also referred to as a circularly polarized light VA mode or a circularly polarized light mode. In contrast, the VA mode using a linearly polarizing plate herein is referred to as a linearly polarized light VA mode or a linearly polarized light mode. As is well known, the circularly polarizing plate is typically formed by combination of a linearly polarizing plate and a λ/4 plate.
Circularly polarized light has a characteristic that the left-handed and right-handed chiralities are changed when the light is reflected on a mirror or the like. For example, when a left-handed circularly polarizing plate is placed on a mirror and light is applied, the light passes through the circularly polarizing plate and is converted into left-handed circularly polarized light, and the light is then reflected on the mirror and converted into right-handed circularly polarized light. The right-handed circularly polarized light cannot pass through the left-handed circularly polarizing plate, so that the circularly polarizing plate is known to have an optical effect of preventing reflection. Such an optical effect of preventing reflection of the circularly polarizing plate prevents unnecessary reflection when a display device is observed in bright environment (e.g. outside). Thus, the circularly polarizing place is known to have an effect of improving the contrast ratio of display devices such as VA-mode liquid crystal display devices in bright environment. The unnecessary reflection herein is considered to occur mainly due to transparent electrodes existing in a display device or metal wiring of TFT elements. If such unnecessary reflection is not prevented, even a display device displaying a substantially perfectly black screen in dark environment provides a large amount of light when displaying a black screen viewed in a bright environment, resulting in reduction in contrast ratio.
As mentioned above, the circularly polarized light VA mode using a circularly polarizing plate can achieve the effect of improving the transmittance and the effect of preventing unnecessary reflection. Still, conventional circularly polarized light VA-mode liquid crystal display devices have a low contrast ratio in inclined viewing angles, and thus fail to give sufficient viewing angle characteristics. Accordingly, such devices require much improvement. Here, various techniques for improving the viewing angle characteristics are proposed using a birefringent layer (phase difference film). For example,    Patent Literature 1 discloses the following method (A);    Patent Literature 2 discloses the following method (B);    Patent Literature 3 discloses the following method (C);    Patent Literature 4 discloses the following method (D); and    Non-Patent Literature 1 discloses the following method (E).
(A) A method using two λ/4 plates satisfying nx>ny>nz
(B) A method using two λ/4 plates satisfying nx>ny>nz and one or two Type-II birefringent layer(s) satisfying nx<ny≦nz in combination
(C) A method using two λ/4 plates satisfying nx>nz>ny and a birefringent layer satisfying nx=ny>nz in combination
(D) A method using the components in the method (C) and one or two λ/2 plate(s) satisfying nx>nz>ny in combination
(E) A method using two uniaxial λ/4 plates (satisfying nx>ny=nz, what is called A plates), a birefringent layer satisfying nx=ny>nz, and a birefringent layer satisfying nx>nz>ny in combination
However, the methods (A), (B), and (C) still require improvement in viewing angle characteristics. The methods (C), (D), and (E) require a biaxial phase difference film satisfying nx>nz>ny (satisfying 0<Nz<1), which is difficult to produce and costs high. The methods require further improvement in this respect.
The present inventors have performed various studies for solving the above problems, and proposed the following method (F) (see Patent Literature 5).
(F) A method using two λ/4 plates, a Type-III birefringent layer satisfying nx=ny>nz, a Type-I birefringent layer satisfying nx>ny≧nz, and a Type-II birefringent layer satisfying nx<ny≦nz
The method (F) improves the viewing angle characteristics by optimally designing the Nz coefficients (parameters representing the biaxiality) of two λ/4 plates. However, it requires further improvement in viewing angle characteristics under the design condition of two general biaxial λ/4 plates satisfying nx>ny≧nz (Nz≧1.0).
Accordingly, the present inventors have further studied to find out the following: by preparing biaxial λ/4 plates satisfying nx>ny≧nz as the two λ/4 plates (first and second λ/4 plates), by adjusting the Nz coefficients thereof to substantially the same values, and by disposing a birefringent layer satisfying nx<ny≦nz at least between the first λ/4 plate and the first polarizer or between the second λ/4 plate and the second polarizer, a circularly polarized light VA-mode liquid crystal display device having a high contrast ratio in a wide viewing-angle range can be easily produced. The present inventors have filed a patent application relating to this method (see Patent Literature 6 and 7).
For production of a circularly polarizing plate, one method is disclosed in which a polarizing plate is produced by a roll-to-roll technique using a λ/4 plate having an in-plane slow axis in the direction diagonal to the flowing direction (machine direction) (for example, see Non-Patent Literature 2). This method allows for adjusting the Nz coefficient of the λ/4 plate to 1.1 to 2.0.
For the circularly polarizing plate, documents disclose that the viewing angle characteristics of a liquid crystal display panel including a liquid crystal layer having a phase difference Δnd of 260 nm can be improved using a λ/4 plate having an Nz coefficient of 2.1, a λ/4 plate having an Nz coefficient of 1.6, a birefringent layer having a phase difference Rth in the perpendicular direction of 300 nm and satisfying nz>nx=ny, and a biaxial phase difference film having an in-plane phase difference of 110 nm and an Nz coefficient of 1.4 (for example, see Non-Patent Literature 3).